The present invention relates to systems for continuously laying marine pipelines and, more particularly, it is directed to reel pipe laying systems having means for straightening the pipe as it is uncoiled.
Marine pipelines are generally buried in the soil on the ocean floor to protect them from strong currents and objects which can damage the pipelines such as anchors or fishing lines. The pipeline is generally lowered to the ocean floor from a barge or some other type of marine vessel.
The pipe is formed in mill length sections which are connected together on the surface and are lowered to the bottom of the ocean as a continuous pipeline from a barge. There are generally two methods of forming and laying the continuous pipe. The first method involves the welding together of the various lengths of pipe on the barge immediately prior to their being introduced into the water. The second method involves welding numerous sections of pipe together and winding them onto a large reel. This reel is then placed on the barge and the pipe is unwound and deposited on the ocean floor. With this type of method, it is necessary to provide some means for straightening the pipe to remove the curvature which is created as the pipe is wound on the reel.
Various systems have been devised for removing the curvature encountered in reel pipe laying systems. For example, U.S. Pat. No. 3,872,680 to Nicholson et al. discloses a five roller system designed to straighten the pipe as it comes off the reel. Four of the rollers are fixed and one roller is movable to create the proper overbend. The entire system is mounted on the deck and is positioned with respect to the reel such that the pipe can be unwound from either the top or the bottom of the reel.
Another deck mounted system for straightening pipe is disclosed in U.S. Pat. No. 3,855,835 to Tisdale, III et al. This patent discloses a system which includes two sets of rollers which are positioned on the deck of the vessel and are adjustable with respect to the position of the reel to reverse bend the pipeline as it is unwound.
U.S. Pat. No. 4,410,297 to Lynch discloses a two roller system in which the rollers are connected to the rotational axis of the reel by a yoke. The roller nearest the reel is mounted on a pair of arms which are controlled by linear actuators which can be adjusted for different pipe diameters and for positioning with respect to successive layers of pipe on the reel. In that system, some adjustments have to be made as successive layers of pipe are removed to maintain the proper departure angle and overbend.
In prior art systems, a substantial amount of force is necessary to pull the pipe from the reel and through the straightening system. This complicates the procedure in those pipe laying systems in which the pipe is laid under tension utilizing an underwater plow to form a ditch and guide the pipeline into the ditch. In underwater plow systems there is a delicate balance between the amount of force necessary to remove and straighten the pipe and the amount of force necessary to create the proper tension in the line to form an appropriate catenary curve in the pipeline without having it extend too far behind the barge before the pipeline reaches the ocean bottom. When the proper balance is achieved, such systems including that of U.S. Pat. No. 4,410,297, offer desirable advantages.
Accordingly, it would be an advancement in the art, including over the system of U.S. Pat. No. 4,410,297 to provide a system for straightening marine pipe as it is unwound from a reel on a barge in which the amount of force necessary to straighten the pipe is reduced and which can easily be adjusted to accommodate different pipe sizes and departure angles. Such a system is disclosed and claimed herein.